The games of tennis, squash, badminton, and racquetball employ racquets having frames that define a generally oval head portion containing strings. The frames are typically made of a composite material such as "graphite", or a metal such as aluminum, and a plurality of string holes are drilled through the head portion to support the ends of the strings. In the case of racquet frames made of composite materials, it is customary to utilize one or more plastic grommet strips along the outer surface of the frame, which include hollow grommet pegs that extend through the string holes in the frame. The purpose of the grommet strips is to prevent direct contact between the strings and the edges of the string holes which, particularly at the points where the strings bend sharply to enter and leave the frame, would otherwise damage the strings.
FIG. 1 shows an example of a tennis racquet having a head portion 10, a pair of converging throat frame members 12, and a handle 14. A throat bridge 15 completes a stringing area which includes a plurality of main strings 16 and a plurality of cross strings 18, the latter being interwoven with the main strings 16. The ends of the strings 16, 18 are contained in grommets as described above, such that the string ends enter and leave the string bed 20 through the hollow barrels of the grommet pegs 22 whose ends project a short distance into the string bed.
Composite materials have a high strength-to-weight ratio, and can be molded into any desired cross-sectional shape, and thus today are the materials of choice for high performance sports racquets. A disadvantage of composite materials, however, is that racquet frames made of composite material can easily be broken by hitting an unyielding surface such as the ground. In order to reduce the incidence of breakage, it is customary in the tip region of the frame to provide the grommet strips with a pair of laterally extending wings to protect the outside surface of the frame. Such grommet strips are normally referred to as bumper strips.
In spite of the protection afforded by conventional bumper strips, frame breakage can still occur upon impact with the ground, particularly in the areas of the string holes where the frame tends to be weakened. Thus, there is always a concern, when designing a new racquet, over the number and placement of the string holes. Too many string holes cause a weakened frame which is more prone to breakage. Too few string holes can result in excess string breakage due to increased string movement and wear, as well as catastrophic string breakage due to having too few strings available to share the forces on the string bed produced by the impact of the ball.
Due to the elliptical shape of the head portion of racquets, in the corners of the head, the string holes for the main strings (longitudinal strings) are interspersed with the string holes of the cross-strings. If the string pattern is not carefully designed, one or more of the cross-string holes can end up being very close to the main string holes. Drilling two string holes in very close proximity is undesirable, because it weakens the frame. Thus, the string pattern needs to be designed to allow sufficient distance between string holes in the corner regions. Alternatively, rather than trying to design a string pattern to ensure adequate separation between the main and cross string holes in the corners, there have been attempts to design the string pattern such that the ends of a cross string and the ends of a main string share the same string holes, thereby reducing the number of holes that need to be drilled through the frame. An example of this is shown in FIG. 2.
FIG. 2 shows three cross strings 18a, 18b, 18c, and three main strings 16a, 16b, 16c, in the upper corner of a racquet having a stringing pattern similar to FIG. 1. The head portion 10 of the racquet frame is formed of a hollow tube of composite material such as to define an outer frame wall 24 and an inner frame wall 26. A plastic grommet strip 28 runs along the outwardly facing surface of the outer frame wall 24, in a stringing groove 30 customarily formed by the outer frame wall 24. The plastic grommet strip 28 includes a plurality of single string grommet pegs 22 and a double string grommet peg 32. As shown, each grommet peg 22, 32 extends through a pair of coaxial string holes formed, respectively, through the inner and outer frame walls 26, 24, and includes an end portion 22a, 32a that projects for a short distance into the stringing area.
As shown in FIG. 2, in a sports racquet, typically a continuous length of stringing forms the cross strings 18a-c and main strings 16a-c. The end of one string, e.g., 18a, exits the string bed and passes through the frame 10 through a grommet hole 32a. Thereafter, string 18a extends along the outer surface of the grommet strip 24 to the next grommet peg 22, whereupon it bends sharply to extend through that grommet peg and the frame 10 to re-enter the string bed. Upon re-entering the string bed, the string 18a interweaves with the main strings 16a-c. By successively passing through adjoining grommet peg holes and crossing the stringing area, the string bed is formed.
The single string grommet pegs 22 are all identical with one another, and are spaced along the grommet strip 24 so as to be aligned with the string holes in the frame, which in turn are located so as to produce the desired spacing between the main and cross strings. The double string grommet peg 32, which contains the shared hole 34, has the same overall configuration as grommet pegs 22, but is typically larger in diameter, in order to have a larger hole 34 for accommodating two strings.
Racquets are generally strung by threading and tensioning the main strings first, followed by the cross strings. Thus, in the example of FIG. 2, the string 16c would be inserted through the shared hole 34 first. Later on in the stringing process, the cross string 18a must be threaded through the same hole 34. Unfortunately, when a main string, e.g. 16c, passes through the grommet 32 and is tensioned, it hugs one wall as it enters the string hole 34, and then hugs the opposite wall as it leaves, crossing the hole 34 in the process. This causes a blockage of the grommet hole 34, and the second string, e.g., cross string 18a, cannot be threaded through the hole 34 without mechanically moving the main string 16c out of the way. This mechanical moving of the diagonally blocking string is carried out by passing a long pointed awl through the shared hole, thus forcing the first string 16c to the side.
If enough force is used, it is possible to maneuver the cross string 18a through the hole 34 and thereafter tension the string 18a without substantial interference from the main string 16c. However, the need to utilize a mechanical crushing force carries a high risk of damaging the string during the stringing process.
Another problem associated with shared holes is that, after the two strings pass through the grommet to enter the string bed, they travel in different directions relative to each other. Thus, as shown in FIG. 2, as the string 16c exits the grommet peg 32, it bends so as to extend parallel to the racquet axis, towards the bottom of the string area. String 18a, in contrast, bends so as to extend perpendicular to the racquet's axis, across the string bed. This tends to spread the lip 32a out in opposite directions, causing tension on the lip 32a. Grommet members tend to be made of nylon, and when this tension becomes too great, the strings 16c, 18a can tear the nylon lip 32a. Once the lip 32a tears, the string can come into contact the sharp edge of the frame string hole, causing breakage.
Shared string holes are desirable due to the fact that they reduce the overall number of holes drilled into the frame, and increase the distance between string holes in the corners of the racquet. It would be desirable to provide a grommet which provides the same advantages, but without the above-described drawbacks of the known shared grommet hole.